Firearms identification - "ballistics".

• Author: Inbau, Fred E.
• Position: Originally published in Journal of the American Institute of Criminal Law and Criminology, vol. 24, p. 825, 1934 - Reprint

"In this land of lies, an ounce of good circumstance is worth many pounds of oral evidence...."(1)

Judicial tribunals usually exercise considerable caution regarding the admissibility into evidence of expert testimony concerning some new phase of our ever increasing wealth of scientific knowledge. In certain instances this attitude has led courts of justice to reach apparently absurd results. The scientist occasionally reads with scorn the judicial denunciation of, or the refusal to consider, the application of a scientific principle or instrumentality. Yet the conscientious scientific investigator himself exhibits the same degree of caution, if not more, whenever there appears the announcement of some alleged discovery or invention.

The wisdom of this judicial conservatism is especially apparent in cases in which scientific testimony might be an important factor in the deprivation of human life or liberty. Nevertheless, there is considerable justification for some of the criticism directed at the courts for their apparently unintelligent treatment of certain cases in which scientific testimony is sought to be introduced.

Frequently the amateurish and unconvincing, and in some instances the deliberately dishonest presentations of "scientific" testimony is responsible for a court's refusal to admit it as evidence. Likewise, the super-cautiousness, or the innate inability of a court to appreciate the significance and reliability of certain forms of scientific evidence even when properly presented often accounts for the unduly deferred recognition of a scientific principle or its application.

Regardless of the justification for excluding evidence of a scientific nature, a court may assume either of two different attitudes--one open-minded and perhaps hopeful of the possibilities of the particular science in question; the other somewhat bigoted and scornfully expressive of the utter hopelessness of scientific aid concerning the problem before the court.(2) Throughout the numerous decisions upon the admissibility of scientific evidence there are many illustrations of both points of view.

In this series of articles an attempt is made to present the status of the criminal law concerning the admissibility of most of the numerous contemporary classes of scientific evidence, with the added effort of briefly explaining the various sciences themselves.

1. FIREARMS IDENTIFICATION--"BALLISTICS"

In ancient times, when it was customary for contracts to be written in duplicate on wooden tablets, there existed the practice of cutting identical notches on the borders of each tablet held by the contracting parties, so that the two documents could be matched and easily verified.(3) In other words, the possibility of an extraneous correspondence of the set of notches appeared to be so remote that this was considered a reliable means of proving the authenticity of such instruments. And strange as it may seem, the science of firearms identification--commonly known as "ballistics"--is based upon the same principle.

It is rather difficult to realize the fact that discharged bullets and shells possess certain characteristic markings which unmistakably identify the weapon in which they were fired--this being determined by comparing the evidence bullet or shell with a test bullet or shell fired from a suspected weapon. Nevertheless, only a superficial understanding of the process of gun manufacture is needed to appreciate the possibility of this phenomenon.(4)

One step in the manufacture of pistols and rifles is the boring of a hole through a cylindrical steel bar and reaming it, after which certain spiral grooves of uniform depth, width, and spacing are cut into the inner surface of the barrel, in order that a bullet fired through the barrel may have imparted to it rotational velocity, either to the right or left, which produces gyroscopic stability and consequently greater accuracy than it could otherwise have.(5) The instrument used for this purpose is known as a "rifling cutter," a sharp tool whose arc-shaped edge, though flawless to the naked eye, when studied under a lens reveals minute saw-like teeth. And when this cutter is scraping out the grooves its serrated edge leaves small scratches on the inner surface of the barrel. Moreover, the same cutter will never leave identical scratches in any other barrel, or in two areas of the same barrel, for that matter, because of the fact that with every stroke the contour of this edge undergoes microscopic but none the less definite changes rendering it incapable of duplicating any given set of markings.(6)

Since the bullet is of a softer metal than the barrel of the gun through which it passes, it naturally receives certain impressions from the irregular surface over which it travels. These constitute the "tell-tale" characteristics which form the basis for the science of firearms identification by a comparison of fired bullets.

The mathematical probability of a duplication of the markings on a fatal bullet by those made upon another bullet fired from a different weapon is so remote as to permit an assumption that it is impossible. The mathematical calculation in this respect may be readily explained by considering the example used by Osborn regarding the possibility of two individuals having only eight distinctive physical characteristics exactly alike (such as a five-inch scar on the right forearm, a mole on the left temple one-half inch in diameter, etc.). Applying Newcomb's formula--that the probability of concurrence of all the events is equal to the continued product of the probabilities of all the separate events--and even with an extremely small fraction representing how frequently each point may be found, Osborn concludes that the possibility of a duplication is one in thirty-eight trillion four billion, or something more than thirty thousand times the total population of the earth.(7) The same principle holds true in the science of firearms identification. So, when a fatal bullet contains not eight but a hundred or more individual and characteristic markings, all matched by a bullet fired from the weapon of an accused individual, it is indeed safe to conclude that only a bullet fired from that particular weapon could duplicate these markings.

Identification may likewise be made from an examination of a discharged shell found at the scene of a crime, whether this be from a shotgun, machine gun, or pistol.(8) The distinctive marks made upon the head of a shell by a firing pin, and by the breech face of the gun afford the evidence for a comparison of fatal and test shells.(9)

The firing pin of a weapon is that part of the mechanism which strikes the shell, causing the explosion. Because it is a machined down bit of steel, its surface presents characteristic scratches which are impressed upon the primers of the shells it discharges. The breech markings also found on the primers and shell heads arise when the empty shell is hurled violently back against the breech face (also a machined and filed surface) by the force of the recoil. Either of the two sets of impressions are distinctive of the particular weapon used and of no other, just as much so, and just as valuable from the standpoint of identification, as bullet markings.(10)

A specially designed instrument--the comparison microscope--is used for the purpose of firearms identification. It consists of two ordinary compound microscope tubes so arranged that images passing through both are brought together in one eye-piece midway between them. Under one of these tubes is placed the fatal bullet shell, and beneath the other the test bullet or shell, forced from the weapon belonging to, or in possession of, the accused. Then, by properly focusing the instrument and moving the bullets or shells into their correct positions, the comparison microscope will transmit the fusion of both fatal and test bullets or shells. If they were fired in the same weapon, there will be found a coincidence, in the case of the two bullets under scrutiny, not only of the major characteristics--the lands and grooves, which after all may only be indicative of the general type of weapon involved--but also of the numerous minute marks described above, which latter features afford unmistakable evidence as to the part, if any, played by the suspected weapon. Similarly, in the case where shells are examined, the presence or absence of a coincidence of the sets of impressions previously described will determine whether or not a particular weapon was used in the commission of the crime under investigation.

After fatal and test bullets or shells have been "matched" under the comparison microscope, enlarged photographs of them may be made for the purpose of illustrating to court and jury the similarities upon which the expert bases his opinion concerning the identity of the weapon used; and often these enlarged photographs assist the examiner in satisfying himself as to the correctness of his conclusions.

No one particular individual can be singled out as the founder of the science of firearms identification. The history of its development is somewhat similar in this respect to that of handwriting and typewriting identification--arising from casual observations and placed upon a scientific basis by several different workers.(11) The early methods were rather crude. For instance, in France (and the same method is widely used there at the present time, despite the availability of the comparison microscope) fatal and test bullets were rolled on tinfoil, and the resulting impressions studied for the purpose of determining their similarity, if any.(12) The magnifying glass was used frequently, and unfortunately even in the United States at the present time so-called "experts" persist in using it. Not until 1925 was firearms identification placed upon a truly scientific basis. In that year, P. O. Gravelle, working in collaboration with C. E. Waite...